Hydraulic power circuit with emergency lowering provisions

ABSTRACT

HYDRAULIC POWER CIRCUIT INCLUDING A LIFT CYLINDER FOR RAISING A LOAD, AND A DIRECTIONAL CONTROL VALVE FOR THE CYLINDER WHICH IS ACTUATED BY A LOW PRESSURE REMOTE CONTROL SYSTEM. THE REMOTE CONTROL SYSTEM NORMALLY RECEIVES MOTIVE FLUID FROM A PUMP, BUT, WHEN THE PUMP IS DISABLED OR IDLE, THE CONTROLS MAY BE ADJUSTED TO EFFECT LOWERING OF AN ELEVATED LOAD UTILIZING THE FLUID UNDER LOAD PRESSURE IN THE CYLINDER. FOR THIS PURPOSE, THE CIRCUIT INCLUDES A SWITCH VALVE WHICH ALTERNATELY CONNECTS THE INLET CONDUIT OF THE REMOTE CONTROL SYSTEM WITH THE PUMP OR THE CYLINDER DEPENDING UPON WHETHER PUMP OUTPUT PRESSURE IS ABOVE OR BELOW A PREDETERMINED VALUE, AND A PRESSURE REGULATOR FOR REDUCING THE PRESSURE OF THE FLUID DELIVERED TO THE REMOTE CONTROL SYSTEM FROM THE CYLINDER. IN APPLICTIONS WHERE THE POWER PUMP FOR THE CYLINDER SERVES ALSO AS THE NORMAL SOURCE OF MOTIVE FLUID FOR THE REMOTE CONTROL SYSTEM, THE PRESSURE REGULATOR MAY BE ARRNAGED BETWEEN THE SWITCH VALVE AND THE REMOTE CONTROL SYSTEM SO THAT IT CAN SERVE TO REDUCE THE PRESSURE OF THE FLUID SUPPLIED TO THAT SYSTEM FROM EITHER THE POWER PUMP OR THE LIFT CYLINDER.

NOV. 16, 197] w. U UF 3,620,129

HYDRAULIC POWER CIRCUIT WITH EMERGENCY LOWERING PROVISIONS Filed July15, 1970 Fi T 18 l/ LOAD T 12 fi L J 14 TO PILOT VALVE INVENTOR FROMHEAD WALDO G- FRUEHAUF END OF CYL. f BY Ai y W ATTORNEYS United StatesPatent Office.

3,620,129 Patented Nov. 16, 1971 3,620,129 HYDRAULIC POWER CIRCUIT WITHEMERGENCY LOWERING PROVISIONS Waldo G. Fruehauf, Kalamazoo, Mich.,assignor to General Signal Corporation Filed July 15, 1970, Ser. No.55,028 Int. Cl. F15!) 11/08, 15/18 U.S. Cl. 91-449 3 Claims ABSTRACT OFTHE DISCLOSURE Hydraulic power circuit including a lift cylinder forraising a load, and a directional control valve for the cylinder whichis actuated by a low pressure remote control system. The remote controlsystem normally receives motive 'fluid from a pump, but, when the pumpis disabled or idle, the controls may be adjusted to effect lowering ofan elevated load utilizing the fluid under load pressure in thecylinder. For this purpose, the circuit includes a switch valve whichalternately connects the inlet conduit of the remote control system withthe pump or the cylinder depending upon whether pump output pressure isabove or below a predetermined value, and a pressure regulator forreducing the pressure of the fluid delivered to the remote controlsystem from the cylinder. In applications where the power pump for thecylinder serves also as the normal source of motive fiuid for the remotecontrol system, the pressure regulator may be arranged between theswitch valve and the remote control system so that it can serve toreduce the pressure of the fluid supplied to that system from either thepower pump or the lift cylinder.

BACKGROUND AND SUMMARY OF THE INVENTION Hydraulic power circuitsemployed on large construction and material-handling vehicles, such asfront end loaders, use high capacity, open center directional controlvalves which require large actuating forces. Therefore, from thestandpoint of reducing operator fatigue, it is desirable that thesevalves be actuated through a low pressure remote control system. Acommon remote control system includes a pair of piloted pressure motorsfor shifting each directional control valve, and a small, manuallyoperated, pressure-graduating pilot valve for each set of motors whichserves to furnish them with pilot fluid at a selectively variablepressure. The pilot valves normally are supplied with operating fluid ata substantially constant, low pressure by a source including a pump anda relief valve. The pump is driven by the propulsion engine of thevehicle, so the actuating cylinders in the power circuit cannot beoperated when the engine is off. Because of this, circuits which includea lift cylinder, such as the boom-lifting cylinders on a loader, shouldbe provided with some means which allows the load to be lowered insituations where the engine stops while the load is in an elevatedposition.

One prior solution to the engineoff lowering problem consists inproviding the lift cylinder with a vent line which leads from itscontracting end to the oil tank, and which is controlled by anauxiliary, manually operated throttling valve. Gradual opening of thisvalve allows controlled escape of oil from the contracting end of thecylinder, and thereby permits the operator to effect and regulatedescent of the load under emergency conditions. While this solution isworkable, it has two disadvantages. First, since the operator normallyeffects lowering of the load through one of the pilot valves, the use ofa separate valve for emergency lowering would be inconvenient and wouldrequire additional training. The second disadvantage is the risk thatthe auxiliary valve will be left open inadvertently and that, therefore,proper operation of the lift circuit will be precluded.

A second, and better, prior proposal for effecting engine-off loweringinvolves use of the oil under load pressure in the cylinder as theoperating fluid for the remote control system. According to thisproposal, the contracting end of the lift cylinder is connected with theinlet conduit of the pilot valves through a pressure reducer, and thedischarge line of the engine driven pump is provided with a check valveoriented to block flow toward the pump. The output pressure of thereducer is comparable to that established by the relief valve;therefore, when the engine is olf and the load is in an elevatedposition, the remote control system still will be supplied withoperating fluid at the proper pressure level. As a result, the operatorwill be able to lower the load in the usual way by merely manipulatingthe appropriate pilot valve. Under normal conditions, i.e., when thepump is running, the pressure reducer must close and isolate the liftcylinder from the remote control system in order to prevent unintendedupward creeping of the load when the pilot and directional controlvalves are in neutral position, and to insure proper operation of thecircuit when these valves are in the power down and raise positions.This, of course, means that the pressure setting of the reducer must belower than the setting of the relief valve. The response characteristicsof the remote control system depend upon the pressure of the fluidsupplied to it, so, if substantially equal response is desired underboth emergency and normal conditions, the settings of the pressurereducer and the relief valve must be fairly close to each other.Therefore, in order to correct for the effects of manufacturingtolerances and of dilferences between installations, either the reduceror the relief valve on each vehicle must be adjusted to make sure thatthe relief valve has the higher setting.

Another drawback of the proposal just mentioned is encountered when thescheme is used in a circuit of the type disclosed in application Ser.No. 37,200, filed May 14, 1970, wherein the main power pump is used asthe normal source of operating fluid for the remote control system. Thenormal supply circuit for the pilot valves includes a pressure reducer,and obviously it would be desirable if this reducer could also serve theengine-off lowering apparatus. This, however, cannot be done with theprior proposal, so its use necessitates inclusion of two pressurereducers.

The object of this invention is to provide an improved engine-offlowering scheme which utilizes the fluid under pressure in the liftcylinder as the emergency source of operating fluid for the remotecontrol system, but which eliminates the need for adjusting thecomponents on a vehicle-byvehicle basis and can be incorporated in thecircuit of application Ser. No. 37,200 without the need for an extrapressure reducer. According to the invention, the power circuit isequipped with a switch valve which responds to the output pressure ofthe engine driven pump and serves to connect the inlet conduit of theremote control system with this pump or with the lift cylinder dependingupon whether the pressure is above or below a predetermined value. Thepredetermined value has no significance as far as performance of theremote control system is concerned, but serves merely as an indicator ofwhether or not the pump is operating normally. Therefore, it can besufficiently lower than the settings of other valves in the circuit toobviate adjustment of any of the values on the vehicle. Moreover, sincethe switch valve, not the pressure reducer, serves to isolate the liftcylinder from the remote control system during normal operation,

the reducer may be located either between the cylinder and the switchvalve, or between the latter and the inlet conduit of the remote controlsystem. The ability to use the last mentioned location is important inembodiments incorporating the teachings of application Ser. No. 37,200,because it allows a single pressure reducer to serve both the normal andthe emergency pilot supply circuits.

BRIEF DESCRIPTION OF THE DRAWING Several embodiments of the inventionare described herein with reference to the accompanying drawing inwhich:

FIG. 1 is a schematic diagram of an embodiment employing separate pilotand main pumps.

FIG. 1A is a schematic diagram showing a modification of the FIG. 1circuit.

FIG. 2 is a schematic diagram of an embodiment wherein the main powerpump serves as the normal source of supply for the remote controlsystem.

DESCRIPTION OF THE EMBODIMENTS OF FIGS. 1 AND 1A Referring to FIG. 1,the improved circuit is employed to actuate a load 11, which mayrepresent the boom of a front end loader, and includes one or more liftcylinders 12, a main power pump 13 and oil tank 14, and an open center,sliding plunger directional control valve 15. Although the completepower circuit normally includes additional actuating cylinders anddirectional control valves, for convenience and clarity the presentdiscussion concerns only the lift portion of the circuit. The plunger ofdirectional control valve 15 is biased to its neutral or hold positionby a centering spring (not shown) and is shifted to its variousactuating positions by a pair of opposed piloted pressure motors 16 and17 which are supplied with operating fluid at a selectively variablepressure by a closed center, manually operated, pressure-graduatingpilot valve 18. The components and mode of operation of a suitableremote control system are fully described in application Ser. No.37,114, filed May 14, 1970, as well as in the co-pending applicationmentioned above.

Pilot valve 18 receives operating fluid at a relatively low pressurefrom either one of two sources. The first includes a relief valve 19 anda small pilot pump 21 which, like main pump 13, is driven by thevehicles propulsion engine. The second source includes the head end 12aof lift cylinder 12, a pressure reducing valve 22, and a relief valve23. Valves 22 and 23 may be packaged together in a unit of the typeshown in application Ser. No. 37,200. In a typical circuit, relief valve19 and pressure reducer 22 are set for a pressure on the order of 200p.s.i., and relief valve 23 is set for a pressure of about 500 p.s.i.Selection between the two sources is effected automatically by a switchvalve 24 which responds to the output pressure of pilot pump 21. Valve24 is biased by spring 24a to the illustrated position, in which itconnects the inlet conduit 25 of pilot valve 18 with the outlet conduit27 of pressure reducer 22, and is shifted by actuating motor 24b to asecond position in which it connects conduit 25 with the dischargeconduit 26 of pump 21. Motor 24!) and spring 24a are so sized that valve24 is held in its second position whenever the pressure in conduit 26 isabove a level indicative of the fact that the pump 21 is running. In thetypical case wherein the remote control system operates at a pressurelevel of 200 p.s.i., motor 24b shifts valve 24 to the second position ata pressure of about 40 p.s.i.

When the FIG. 1 circuit is in service and the engine is running, thepressure in conduit 26 will be well above the setting of valve 24, sothis valve will assume its second position, and pump 21 will deliveroperating fluid to pilot valve 18 at a pressure determined by thesetting of relief valve 19. Under this condition, conduit 27 is isolatedfrom both of the conduits 25 and 26, so there will be no flow from pump21 to the head end 12a of cylinder 12 or from the head end to the remotecontrol system, Because of this, the settting of reducer 22 may behigher than the setting of relief valve 19 without adverse effect.

It will be noted that the relief valve 23 is in communication with thehead end 12a of cylinder 12 through pressure reducer 22, and thattherefore some oil can escape from the cylinder to tank 14 at times whenthe head end pressure is higher than the relief valve setting. However,since the setting of relief valve 23 is higher than the setting ofreducer 22, the reducer will be closed when the load pressure is thishigh, and consequently the volume of oil which escapes from the cylinderwill be limited to the small amount which leaks across the spool of thereducer 22 from conduit 28 to conduit 27.

As shown in FIG. 1A, pressure reducer 22 and relief valve 23 may beconnected into the circuit between switch valve 24 and the inlet conduit25 of the remote control system, rather than between the switch valveand the lift cylinder. Although, in this embodiment, the switch valve 24may serve to connect the inlet conduit 29 of reducer 22 with either thedischarge conduit 26 of pump 21 or the conduit 28 leading from the headend of the lift cylinder, this would require that all of the oildelivered to the remote control system during normal operation passthrough reducer 22. Since the pressure reducer is needed only duringemergency conditions, the pressure losses accompanying flow through thisdevice would constitute a waste of energy. Therefore, it is preferredthat the switch valve 24 in the FIG. 1A embodiment deliver oil from pump21 to conduit 25 through a conduit 31 which by-passes reducer 22.

Inasmuch as the relief valve 23 in FIG. 1A is isolated from the liftcylinder whenever the pilot pump 21 is running, this version of theinvention inherently precludes escape of any oil fro-m the head end ofthe cylinder to tank through that relief valve. The embodiment of FIG.1A also is desirable because it allows the valves 22-24 to be packagedas a unit which, except for minor changes in circuitry, can be employedin power circuits which use the main power pump as the normal source ofoperating fluid for the remote control system.

DESCRIPTION OF THE FIG. 2 EMBODIMENT FIG. 2 shows the invention embodiedin the power circuit of application Ser. No. 37,200. In this circuit,the supply line 32 leading from power pump 13 to directional controlvalve 15' is provided with an adjustable throttling valve 33, and thepilot valve 18 normally is supplied with oil through a branch line 34which joins the supply line at a point upstream of the throttling valve.Valve 33 is controlled primarily by apparatus including a spring 33awhich bias it toward a high flow-restricting position, and an opposingfluid pressure motor 33b which is connected with sup-ply line 32 througha restriction 35 and with tank 14 through a vent valve 36 operated inunison with pilot valve 18. The arrangement is such that valve 33assumes a low flow-restricting position, and imposes a minimumbackpressure on pump 13, when the pilot valve is in neutral position,and shifts to the high flow-restricting position and creates abackpressure sufficient to operate the remote control system when pilotvalve 18 is shifted away from neutral position to effect actuation ofcylinder 12. Valve 33 also is equipped with an override motor 33c whichshifts it to the low flow-restricting position where the load pressurein line 32 developed during actuation of the cylinder is itself highenough to insure proper operation of the remote control system.

The pressure of the oil delivered to branch line 34 varies with the loadpressure in conduit 3-2, so, under high load conditions, it must bereduced before the oil reaches pilot valve 18. In accordance with theinvention, this function may be performed by the same reducer 22 whichserves the emergency engine-otf lowering scheme if, as illustrated, thiscomponent is located downstream of switch valve 24. The arrangement issimilar to the one shown in FIG. 1A except, of course, that the bypass31 is omitted and valve 24 is used to connect one or the other of thelines 28 and 34 with the reducer 22.

During normal operation, i.e., when pump 13 is running, the pressure insupply line 32 upstream of valve 33 will always be higher than thesetting of switch valve 24 regardless of the position of throttlingvalve 33. Therefore, valve 24 will connect line 34 with pressure reducer22, and the remote control system can be operated in the usual wayutilizing a portion of the out put of pump 13 as the operating fluid. Onthe other hand, when the engine is off, the pressure in line 34 willdissipate, and valve 24 will shift to the illustrated position andconnect line 28 with the pressure reducer. Therefore, if load 11 is inan elevated position, pilot valve 18 will still receive oil at thepressure needed for proper operation of the remote control system, andemergency lowering of the load can be effected as in the otherembodiments.

What is claimed is:

1. In a hydraulic power circuit including a lift cylinder (12) forraising a load (11), directional control valve means (15) forcontrolling flow from a power pump (13) to the cylinder and from thecylinder to a reservoir (14), a low pressure remote control system(1648) for actuating the directional control valve means and having aninlet conduit (25) through which it receives motive fluid, and pumpingmeans (21 or 13) for delivering fluid under pressure to the inletconduit, the improvement which comprises emergency means for permittinglowering of the load from an elevated position when the pumping means isdisabled, the emergency means including (a) an auxiliary conduit (28)for leading fluid expelled from the cylinder (12) by the load to theremote control system;

(b) switch valve means (24) responsive to the output pressure of thepumping means for alternately connecting the inlet conduit (25) with thepumping means (21 or 13) or the auxiliary conduit depending upon whetherthe pressure is above or below a predetermined value; and

(c) regulating means (22) for reducing to a predetermined low level thepressure at which fluid is delivered to the remote control systemthrough the auxiliary conduit.

2. The improved power circuit defined in claim 1 in which the regulatingmeans (22) is interposed in the auxiliary conduit (28) between theswitch valve means (24) and the lift cylinder (12).

3. The improved power circuit defined in claim 1 in which (a) the powerpump (13) is connected to the directional control valve means (15)through a supply passage (32) containing a throttle valve (33) which isadjusted automatically in response to actuation of a pilot valve (18) inthe remote control system;

(b) the switch valve means (24-) connects the inlet conduit (.25) withthe supply passage (32) at a point upstream of the throttle valve whenthe output pressure of the power pump is above said predetermined valve,whereby said power pump serves as said pumping means; and

(c) said regulating means (22) is interposed between the switch valvemeans (24) and said inlet conduit (25), whereby this means (22) servesto reduce the pressure of the fluid delivered to the inlet conduitthrough the auxiliary conduit (28) as well as the pressure of the fluiddelivered thereto by the power pump (13).

References Cited UNITED STATES PATENTS 2,157,707 5/1939 Keel 91-449 X3,376,793 4/1968 Papadia et al. 91-446 X 3,486,333 12/1969 Thomas -52 HEEDGAR W. GEOGHEGAN, Primary Examiner US. Cl. X.R.

